Well there is a lot of flaky crap in iOS 15, so it is could be a bug if there isn't an obvious option set incorrectly. I'm running iOS 15.0.2 on my iPhone XR and here is a screen capture after I sent two photos to myself. One was viewable as soon as I opened the email, the other had to be tapped to load but did load. I am of course on my own VIP list and I have the protect Mail Activity option turned OFF. I do have Block All Remote Content set to ON though.
Is the email from a known contact? Is it from an unknown contact or one that is not in the VIP list?